1. Field of the Invention
The present invention relates to the uniform coating or encapsulation of energetic materials that are suitable for controlled release particulate applications, including air bags, gas generators, solid propellants, ordnance and time release drugs. In another aspect, the invention relates to a green technology.
2. Description of Related Art
The development of advanced, coated, particulate materials with tailored characteristics of release onset and rate control has direct application to air bags, gas generators, solid propellants, ordnance and time release drugs. These goals have been a long-lasting challenge to the propellant, ordnance and pharmaceutical industries. Until the advent and development of supercritical-fluid-aided material processing, the effort to identify successful techniques for achieving the required microns-thin, high-quality particle coatings was unsuccessful.
In the last ten years or so, there has been an influx of new supercritical-fluid-aided material processing options for a variety of materials. These processing applications include impregnation of porous matrices, formation of controlled porosity materials, coating/spraying of two-dimensional flat surfaces, macro-encapsulation, extrusion, nucleation of particles with narrow and controlled particle size, and drying.
Micro-encapsulation using supercritical fluids (SCF) has been tried by Tom et al. [1993] for drug-polymer systems with very limited success due to poor understanding of complex phenomena involved in co-nucleation of chemically diverse components. The RESS (Rapid Expansion of Supercritical Fluid Solutions) process used in these trials (first introduced by Krukonis [1984]) for micro-encapsulation has major limitations. Not only do particles with desired particle size and shape have to be nucleated, but also a uniform encapsulating material has to be formed simultaneously. The alternatives to SCF micro-encapsulation processes are spray chilling, standard fluidized bed coating, centrifugal extrusion, coacervation, and co-crystallization. These alternative processes often require the use of surfactants and thermally sensitive compounds that should be avoided because of their potential for contamination and disruption of the desired thin films.
There are many techniques, including SCF nucleation, which result in formation of particles with desired shape and particle size distribution. However, there are no processes other than SCF techniques that permit coating of particles with high quality of thin films. This invention permits coating and micro-encapsulation of particles in an environment fluidized by a supercritical fluid.
In accordance with the present invention, there is provided a method for forming thin films on particles, the method including the steps of suspending particles to be coated in a supercritical fluid containing dissolved coating material therein and then simultaneously nucleating and depositing the dissolved coating material onto the surface of the particles forming the desired thin films.